This application claims the benefit of Korean Patent Application No. 2003-37739, filed on Jun. 12, 2003, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a method for crystallizing amorphous silicon, and more particularly, to a method for crystallizing amorphous silicon using a sequential lateral solidification (SLS) process.
2. Discussion of the Related Art
Recently, as modem society moves quickly toward an information-oriented society, flat panel displays, which have many advantages such as slimness, lightweight, low power consumption and the like, are widely used. In particular, among the flat panel displays, liquid crystal displays (LCDs), which have a superior color reproduction, have been actively developed.
Generally, an LCD includes two substrates facing each other. Electrodes are formed on the facing surfaces of the two substrates, and a liquid crystal material is injected into a space defined between the two substrates. The liquid crystal display (LCD) controls light transmissibility using electric fields applied between the electrodes to display an image corresponding to video signals.
The lower substrate of the LCD includes thin film transistors. The active layer of the thin film transistors is generally formed of amorphous silicon (a-Si:H). As the amorphous silicon can be deposited as a thin film at a relatively low temperature, it is widely used for forming a switching device of liquid crystal panels. The substrate may be made of a glass having a relatively low-melting point. However, the amorphous silicon thin film has a problem in that it harms the electrical characteristics and reliability of the switching device of the liquid crystal panels, and causes difficulty when increasing the screen size of the LCD.
As large-size and high-definition laptop computers and wall-mountable LCD TVs integrated with image driver circuits are commercialized, pixel-driving devices are required to have improved characteristics, such as a high electric field effect mobility (30 cm2/VS), a high frequency performance characteristic, and a low leakage current. To improve the characteristics, a high quality poly-crystalline silicon is required. The electrical characteristics of the poly-crystalline silicon thin film particularly depend on the size of grains. That is, the greater the size of the grains, the higher the electric field effect mobility.
Accordingly, a method for single-crystallizing silicon has become a major issue in the art. PCT Publication No. WO 97/45827 and Korean Published Patent No. 2001-004129 disclose a sequential lateral solidification (SLS) technique for making a massive single crystalline silicon structure by inducing lateral growth of a silicon crystal using a laser as an energy source. The SLS technique has been developed based on the fact that silicon grains grow in a direction normal to the boundary surface between liquid silicon and solid silicon. SLS techniques crystallizes an amorphous silicon thin film by making silicon grains grow laterally to a predetermined length, by appropriately adjusting energy intensity and beam projection range of a laser. Such a method for crystallizing amorphous silicon using the SLS technique will be described hereinafter in conjunction with the accompanying drawings.
FIG. 1 shows an SLS apparatus used for a crystallizing method using the SLS technique. A SLS apparatus 100 includes a laser generator 111 for generating a laser beam 112, a convergence lens 113 for converging the laser beam 112 irradiated from the laser generator 111, a mask 114 for dividing the laser beam into a plurality of sections and projecting the divided sections on a substrate 116, and a scale lens 115 for reducing the laser beam 112 passing through the mask 114 to a predetermined scale.
The laser generator 111 emits the laser beam 112, and the intensity of the emitted laser beam 112 is adjusted while passing through an attenuator (not shown). The laser beam 112 is then directed onto the mask 114 through the convergence lens 113. The substrate 116 having an amorphous silicon layer deposited on its surface is disposed on an X-Y stage 117, corresponding to the mask 114. At this point, in order to crystallize the entire area of the substrate 116, a method for gradually enlarging the crystallized area by minutely moving the X-Y stage 117 is used. The mask 114 is divided into laser beam transmission regions 114a allowing for the transmission of the laser beam 112, and laser beam interception regions 114b for absorbing the laser beam 112. The distance between the transmission regions 114a (the width of each interception region 114) determines length of grains laterally grown.
A method for crystallizing amorphous silicon using the above-described SLS apparatus will be described hereinafter. Generally, crystalline silicon is used for forming a buffer (insulating) layer (not shown) on the substrate 116, and amorphous silicon is deposited on the buffer layer. The amorphous silicon layer is deposited on the substrate 116 using, for example, a chemical vapor deposition (CVD) process, during which a large amount of hydrogen can be retained in the amorphous silicon layer. Since the hydrogen retained in the amorphous silicon tends to separate from the thin film in the presence of heat, a heat treatment is followed for a dehydrogenization process. That is, if the hydrogen is not removed in advance, the crystallized layer may be exfoliated due to the rapid volume expansion of the hydrogen gas retained in the amorphous silicon layer in the course of the crystallization process.
When performing the SLS crystallization, it is difficult to crystallize the entire area of the surface at once. That is, since a width of the laser beam 112 and a size of the mask 114 are limited, the single mask 114 should be realigned many times, and the crystallization process should be repeated whenever the mask 114 is realigned, to crystallize a large-sized screen panel. At this point, it should be understood that an area that is crystallized, which is as large as the area of the single mask 114, is a unit block, and that the crystallization of the unit block should be realized by repeatedly irradiating with the laser beam.
FIG. 2 shows a schematic plane view illustrating a mask used for the SLS technique. As shown in the drawing, a mask 114 comprises patterned transmission and interception regions 114a and 114b. Each of the transmission regions 114a is defined by a longitudinal slit extending in a first direction. At this point, a width of the transmission region 114a should be less than or equal to twice as long as a maximum length of the grain grown by a first laser irradiated process. When the first beam is directed onto the mask structure as in the above, grains grow laterally in the melted regions of the amorphous silicon layer, which correspond to the transmission regions of the mask. In particular, the grains grow laterally from the both boundaries of the melted region until they contact each other at a middle line of the melted region.
In the course of the crystallization process, the laser beam pattern after passing through the mask 114 and being reduced by the scale lens 115 (see FIG. 1) moves in a direction of an X-axis. At this point, the crystallization process proceeds while the laser beam pattern moves from hundreds of xcexcm to several mm (i.e., length of the pattern reduced by the scale lens 115) in the X-axis direction.
The crystallization method using an SLS technique will be described in more detail hereinafter with reference to FIGS. 3A to 3C, which illustrate an example of a two-shot SLS poly-silicon crystallization method. In this example, three transmission patterns (regions) are defined on the mask.
In the two-shot poly-silicon crystallization method, the regions of the amorphous silicon layer that correspond to the transmission regions are crystallized by irradiating the laser beam twice. In addition, this crystallization process is consecutively carried out in a lengthwise direction. When the crystallization is completed in the lengthwise direction of the substrate, the laser beam pattern moves minutely in a widthwise direction, and then moves lengthwise to proceed with the crystallization, thereby completing the crystallization process for a desired region.
In more detail, the mask 114 (see FIG. 2) is first located corresponding to the substrate, and a first laser beam is used to proceed with the crystallization process for the amorphous silicon layer deposited on the transparent insulating substrate. At this point, the laser beam is divided into a plurality of sections while passing through the plurality of slits 114a (see FIG. 2) formed on the mask 114. Regions of the amorphous silicon layer, which correspond to the slits 114a, are liquefied by the divided sections of the first laser beams. In this case, laser energy intensity is set to a complete melting regime in which the amorphous silicon layer completely melts. When the laser beam irradiation is completed, silicon grains are laterally grown at a boundary between the solid amorphous silicon region and the liquefied amorphous silicon region.
At this point, a width of the beam pattern passing through the mask is set to be less than or equal to twice the length of the grain grown. In addition, the crystallized regions correspond to the transmissive regions 114a (see FIG. 2) of the mask. Therefore, each of the crystallized regions A1, A2, and A3 has a length identical to that of each of the transmissive regions 114a. The regions of the amorphous silicon layer, which correspond to the interception regions 114b (see FIG. 2) of the mask, remain as amorphous silicon regions 167. In the crystallized regions A1, A2, and A3, the grains 166a and 166b are laterally grown from the boundaries between the liquefied silicon and the solid silicon, thereby defining a grain boundary as shown in FIGS. 3A to 3C.
Afterwards, the crystallization process is consecutively carried out in a direction of the X-axis while the stage on which the substrate is disposed moves from hundreds of xcexcm to several mm, which is the same as a length of the mask pattern (beam pattern). As shown in FIG. 3B, when the crystallization in the X-axis direction is completed, the mask 114 or the X-Y stage 117 (see FIG. 2) moves minutely in the direction of a Y-axis.
Next, a second laser irradiation initiates when the first crystallization is finished in the direction of the X-axis. Through the second laser irradiation, the grains of the crystallized silicon formed by the first laser irradiation are further consecutively grown. That is, the grains are further grown to have a length that is half as long as a distance xe2x80x9ckxe2x80x9d, which is the distance between the grain boundary 116c of the crystallized region A1 and the grain boundary of the adjacent crystallized region A2. Accordingly, as shown in FIG. 3C, a poly-silicon thin film formed of the grains 168a and 168b having a predetermined length can be realized. At this point, in newly crystallized regions B1 and B2, the grains 168a and 168b are vertically grown from the boundaries between the liquefied silicon and the solid silicon. The grains 168a and 168b are further grown until they contact each other, thereby defining a new grain boundary 168c. FIG. 3 shows an enlarged view of such grains and boundary.
FIG. 4 is a schematic view illustrating a process in which a protruding portion forms at a grain boundary in the course of a crystallization process when using the SLS technique. As shown in the drawing, liquefied silicon 192 that is melted by a laser beam transforms to solid silicon 193 during a lateral crystallization process. At this point, as a mass transfer occurs in a lateral direction, a protruding portion 191 is formed at a grain boundary 190. Since the density of liquefied silicon is greater than that of solid silicon by about 10% (density of liquefied silicon is 2.30 g/cm3, and the density of solid silicon is 2.53 g/cm3), the volume of liquefied silicon expands by 10% when it transforms into solid silicon.
Accordingly, when the grains are grown by the SLS technique, the mass transfer to the middle portion of the liquefied portion occurs, as the solid portion is grown. Furthermore, the finalized shape of the solid portion has the protruding portion 191 formed at the grain boundary 190. The protruding portion 191 causes the crystalline silicon layer to have a coarse surface, making it difficult to form a thin film transistor with a high electric charge mobility and thereby harming the reliability of the thin film transistor.
Furthermore, since some of the grain boundaries of the poly crystal silicon may function as a barrier for obstructing current flow, it is difficult to provide a reliable thin film transistor device. In addition, electrons may collide with each other at some of the grain boundaries generating a collision current, causing the insulating layer to deteriorate or even to break, resulting in an inferior device.
Accordingly, the present invention is directed to a method for crystallizing amorphous silicon that substantially obviate one or more problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide a method for crystallizing amorphous silicon that can reduce the height of a protruding portion formed at a grain boundary of a poly-silicon layer in an sequential lateral solidification technique process.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a method for crystallizing amorphous silicon may, for example, include preparing a substrate on which an amorphous silicon layer is formed; aligning a mask above the substrate, the mask being divided into first and second blocks, the first block having a plurality of first transmission slits and a plurality of interception portions formed between the first transmission slits, the second block having a plurality of second transmission slits alternately arranged with the first transmission slits and a plurality of third transmission slits formed corresponding to middle portions of the first transmission slits; forming first crystalline regions on the amorphous silicon layer by irradiating a laser beam through the first transmission slits, each of the first crystalline regions having a crystallized region and a nucleation region; and crystallizing non-crystalline regions between the first crystalline regions and re-crystallizing the nucleation regions by moving either or both of the substrate and the mask by a distance and by irradiating a laser beam through the second and third transmission slits.
According to another aspect of the present invention, a mask used for crystallizing an amorphous layer into a poly-crystal layer using a sequential lateral solidification technique may, for example, include first and second blocks patterned side by side, the first and second blocks having an identical size to each other, wherein the first block comprises a plurality of first transmission slits and a plurality of interception portions formed between the first transmission slits; and the second block comprises a plurality of second transmission slits alternately arranged with the first transmission slits and a plurality of third transmission slits formed corresponding to middle portions of the first transmission slits.
It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.